CppEditor: Verify proper highlighting of the concept keyword

Change-Id: I1c814da472f10539f87640a52822b188b9b7a2cd
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/testcases/highlightingtestcase.cpp

@@ -25,3 +25,5 @@ template<int n = 5> class C;
 struct ConversionFunction {
     operator int();
 };
+
+template<typename T> concept NoConstraint = true;